Synthesis, characterization and optical properties of Eu 2 O 3 mesoporous thin films This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Nanotechnology 18 055705 (http://iopscience.iop.org/0957-4484/18/5/055705) Download details: IP Address: 132.174.255.49 The article was downloaded on 29/04/2013 at 10:37 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Synthesis, characterization and optical properties of Eu2O3 mesoporous thin films
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2007 Nanotechnology 18 055705
(http://iopscience.iop.org/0957-4484/18/5/055705)
Download details:
IP Address: 132.174.255.49
The article was downloaded on 29/04/2013 at 10:37
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
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Synthesis, characterization and opticalproperties of Eu2O3 mesoporous thin filmsYolanda Castro1, Beatriz Julian1, Cedric Boissiere1, Bruno Viana1,Heinz Amenitsch2, David Grosso1 and Clement Sanchez1
1 Laboratoire de Chimie de la Matiere Condensee UMR-CNRS 7574, Universite Pierre etMarie Curie, 4 place Jussieu, 75252 Paris, France2 Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences,Steyrergasse 17/VI, 8010 Graz, Austria
Received 11 September 2006, in final form 15 November 2006Published 9 January 2007Online at stacks.iop.org/Nano/18/055705
AbstractFor the first time, nanocrystalline Eu2O3 thin films, exhibiting ∼50% ofporous volume, highly accessible mesoporosity, pore diameter of about10 nm, and thermal stability up to 830 ◦C, were prepared using theevaporation-induced self-assembly (EISA) process from europium (III)chloride and specific polyethylene oxide based copolymer structuring agent.Crystallization of the inorganic network was achieved through a controlledthermal treatment sequence that was followed by in situ time-resolvedSAXS/WAXS analysis involving synchrotron radiation. Atomic forcemicroscopy (AFM) and x-ray diffraction (XRD) were used to complete thecharacterization of the crystallization of the film. Porous structures wereadditionally analysed by transmission electron microscopy (TEM), andenvironmental ellipsometry porosimetry (EEP), while the composition of thefilms was characterized by Rutherford back-scattering (RBS). Finally, theluminescence properties of these layers were investigated.
1. Introduction
In the last 20 years, important improvements in the preparationof novel mesostructured and mesoporous materials withvarious pore organizations and dimensions [1] followed thediscovery of the M41S family by Mobil researchers [2, 3].In order to obtain these materials under thin films, theevaporation-induced self-assembly (EISA) process has provento be well adapted and is now commonly applied to prepareSiO2, or crystalline metal oxide meso-ordered porous opticallayers on various types of surfaces. Conditions of preparationof such novel materials have been apprehended for SiO2
and TiO2 systems in detail by our group during the last5 years [4, 5], which allows fast transposition to the elaborationof additional oxide matrices. To name a few, crystalline meso-ordered thin films of ZrO2 [6], Al2O3 [7], CeO2 [8], SrTiO3 [9],and SnO2 [10] have been prepared through this generalapproach. Moreover, the high surface area and accessibilityassociated with these mesostructures make them potentiallyinteresting in multiple domains of application (e.g. sensors,solar cells, catalysis, energy conversion, microelectronics, etc).
The preparation of nanocrystalline rare earth oxidestructures have gained considerable interest for applications inthe fields of catalysis, magnetism and optics. However, exceptfor CeO2, to the best of our knowledge, their preparation asmesostructured films has not been reported. It is importantto notice that the chemical properties (oxidation state, radii,acidity, coordination number, etc) of the lanthanide family arequite different compared to those of transition metal ions, asa result of their electronic configuration. Thus, the control ofthe physico-chemical processes in these systems would opennew strategies for preparation of mesoporous materials basedon rare earth ions by the EISA process.
Among lanthanides, europium presents characteristicemission features associated with its intrinsic 5D → 7Ftransitions [11] and is already used as a phosphor activatorin colour cathode-ray tubes and in liquid-crystal flat displays.Eu3+ luminescence is well known and depends strongly on thechemical environment of the luminescent centres. Since itsfluorescence can be used as a probe for the material structure,Eu2O3 has been chosen in this study as a reference material,even though the optical response is expected to be poor due
to the high lanthanide concentration. The preparation of suchan organized porous network as a thin film can be extended toother rare earth oxides, opening new ways in the developmentof novel properties and advanced applications.
This work reports the preparation of the first mesoporousnanocrystalline rare earth (Eu2O3) thin films by the EISAmethod combined with a careful thermal treatment. Inthe first stage, the synthesis process and conditions usedin the preparation of the precursor solution and filmsare described. Several analysis techniques such assimultaneous SAXS/WAXS (synchrotron radiation), highresolution transmission electron microscopy (HRTEM), atomicforce microscopy (AFM), x-ray diffraction (XRD) andRutherford back scattering (RBS) were used to characterizethe films. Environmental ellipsometry porosimetry (EEP)and fluorescence spectroscopy were used to characterizethe material formation, structure, composition and opticalproperties.
2. Experimental details
Europium oxide films were prepared via dip-coating usinga solution prepared in two steps. Europium chloride hex-ahydrate, 99.9% (Aldrich), was mixed with absolute ethanoland deionized water in a molar ratio 1EuCl3:30H2O:40EtOH.Amphiphilic poly(ethylene-co-butylene)-b-poly(ethylene ox-ide) block copolymers (79 and 89 units respectively, calledKLE22) were mixed with absolute ethanol in a relation of 0.3 gKLE22/1 g EtOH. This mixture was maintained under stir-ring up to complete dissolution of the copolymer before be-ing added to the first solution and maintained under stirringfor 30 min. After ageing the solution for 1 day, films wereprepared by dip-coating on silicon wafer substrates at a con-stant withdrawal rate of 4 mm s−1, under a controlled rela-tive humidity (RH) of 10% up to complete evaporation, fol-lowed by a few minutes at 60% RH. After deposition, the filmswere quickly transferred at 100 ◦C in order to force the stiff-ness of the network. This latter step aims at quenching thediffusion of uncondensed cations, and more precisely preventscrystallization of EuCl3 at the film surface. The eliminationof the copolymer and the consolidation and crystallization ofthe porous films was achieved by heat treatment in air throughthe sequence 250 ◦C/30 min, 350 ◦C/30 min, 400 ◦C/30 min,500 ◦C/15 min, 600 ◦C/10 min, 680 ◦C/5 min using a heatingramp of 10 ◦C/min and for some samples an additional heatingof 830 ◦C/5 min. Non-porous reference films for EEP investi-gation were prepared by deposition of the surfactant KLE22-free solution.
SAXS/WAXS simultaneous analyses were performed atthe Austrian SAXS beam line of Elettra (Italy) [12] throughthe set-up described elsewhere [13]. Films were previouslystabilized at 300 ◦C in air for 2 h. Data were collected on alinear detector (60 s acquisition) for the WAXS analysis, whileSAXS patterns were collected on a bi-dimensional detector(20 s acquisition). The incident x-ray beam angle with thesample surface plane was fixed at 5◦.
High resolution transmission electron microscopy(HRTEM) images were obtained using a JEOL 100 CX IIIinstrument. Determination of the film composition was as-sessed using Rutherford back-scattering (RBS) (2.5 MV Van
Figure 1. Simultaneous ((a)–(c)) SAXS and (d) WAXS analyses ofEu2O3 mesostructured films during thermal heating in air at10 ◦C min−1.
de Graaf accelerator and a normal incidence beam of 4He+ ions2.0 MeV). The profiles of the surface structure of the sampleswere recorded with an AFM of Digital Instruments (DI-CPII,Vecco) in non-contact mode, using an etched silicon probe with<10 nm Tiproc (ref. MPP11120 from Vecco). These samefilms were characterized by XRD (Bruker D8). Measurementsof refractive index and thickness were performed as a func-tion of RH using a variable angle spectroscopic ellipsometer(Equipment Woollam M2000U) equipped with a humidity con-trolled chamber. Finally, luminescence properties of the Eu2O3
films after heat treatment at 680 and 830 ◦C were investigatedat room temperature. The excitation at the 5D2 level (∼460 nm)of the Eu(III) ion was generated with an OPO (optical para-metric oscillator) pumped by the third harmonic of a ThalesQ-switched Nd:YAG laser. An ICCD Roper Scientific cam-era Jobin–Yvon monochromator was used to measure the fluo-rescence and the time-resolved luminescence. Lifetime valueswere extracted from the shape of the decay profiles.
3. Results and discussion
In situ 2D-SAXS patterns of these films are given infigures 1(a)–(c) before and after crystallization. Simultaneouscorresponding WAXS diagrams have been plotted versustemperature of treatment. At 300 ◦C (figure 1(a)), an elongateddiffraction ring confirms the existence of arranged domains andmeso-ordering in the film. On this signal, discrete diffractionpoints are not observed. This is characteristic of poorlyordered domains that are randomly orientated with respect tothe substrate. The correlation distances measured in the normaland parallel diffraction directions with respect to the surface ofthe film are 9.5 and 25 nm respectively. These values suggest
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Nanotechnology 18 (2007) 055705 Y Castro et al
5 nm
Figure 2. HRTEM image of a mesoporous crystalline Eu2O3 filmcalcined at 680 ◦C.
that the film structure has shrunk considerably (i.e. 60%)normal to the surface. The WAXS 3D diagram (figure 1(d))reveals that crystallization eventually occurs above 600 ◦Cunder the present conditions.
The WAXS evolution diagram shows a peak at s =3.9 nm−1 (d = 2.56 A, labelled as *) with low intensity thatdisappears just before crystallization at T = 600 ◦C. Thispeak can be indexed as the (102) plane of the EuOCl structure(JCPDS No 12-163). This indicates that chloride remains inthe system until its removal as volatile species after calcinationat around 600 ◦C. Rutherford back-scattering (RBS) analysisof samples prepared at 400 ◦C confirms the presence of 10% ofCl atoms in the structure, while no Cl atoms were detected onfilms annealed above 650 ◦C due to the oxidation of remainingchloride species at 550 ◦C. The crystallization of Eu2O3 startsat 600 ◦C, as revealed by the presence of two peaks associatedwith the (222) (d = 3.1 A) and the (400) (d = 2.7 A) ofthe corresponding body-centred cubic structure (JCPDS No34-392). Two regimes seem to exist in the crystallization ofthe mesoporous network as the peak morphology follows asequential evolution: (i) a classical growth in intensity between600 and 650 ◦C, which is followed by a steady state period,and (ii) an increase in intensity combined with a narrowingof both peaks above 750 ◦C. These observations suggest thatparticles nucleate and grow to a critical size by consuming thesurrounding parent materials up to 650 ◦C. The network is then
unchanged up to 750 ◦C, above which diffusive sintering ofintermediate particles leads to the formation of larger particles.Particle size determination could not be done here as a resultof the experimental geometry and the large x-ray beam size(0.6 mm in diameter) [13]. SAXS diagrams corresponding toboth states of crystallization are also reported, and show thetypical intensity increase of diffracted signals correspondingto perpendicular-to-the-surface planes, accompanied by theloss of signals that correspond to parallel-to-the-surface planes(figures 1(b), (c)). This 2D-SAXS evolution has been attributedto typical matter migration associated with diffusive sintering,leading to pore fusion. In the particular case of an initialbody-centred cubic (bcc) mesostructure, the resulting structureadopts a grid-like morphology with fully open porosity asshown by EEP analysis (figure 5) [13]. Here, in view ofthe SAXS data, we can safely assume a similar mechanism,which in this case leads to a periodical bidimensional porousnetwork standing normal to the substrate surface with a d-spacing of 25 nm (value extracted from figure 1(b)) at 680 ◦C.This behaviour has been observed in other structures withbetter organization such as SrTiO3 or MgTa2O6 [9]. Whenthe temperature increases above 830 ◦C, diffraction signalsdisappear, indicating that the structure loses its integrity andeventually collapses as a result of the extended diffusivesintering.
Figure 2 shows a high resolution transmission electronmicroscopy (HRTEM) image of Eu2O3 film treated at 680 ◦C.This image is representative of the whole sample after templateelimination and Eu2O3 crystallization, and it confirms thepresence of nanometric crystals. In particular, this figureshows the crystallographic reticular plane (222) of the Eu2O3
body-centred cubic structure. A similar morphology wasobserved with films treated at 830 ◦C except that wall thicknessand cohesion seem to have slightly increased upon such atemperature increment. Considering the 2D-SAXS pattern at830 ◦C, one expects the correlation distance to be retained,suggesting that TEM images must represent view planes wherethis periodicity cannot be observed.
The morphologies of the heat treated (680 and 830 ◦C)mesoporous Eu2O3 films were investigated by AFM analyses.Figure 3 shows the AFM images taken over an area of500 nm×500 nm for both film surfaces. The vertical roughnesswas determined to be 1.2 and 3 nm respectively, indicating high
500 nm
0 nm
250 nm
500 nm0 nm 250 nm
0.00 nm
23.39 n500 nm
0 nm
250 nm
500 nm0 nm 250 nm
0.00 nm
12.40 nm
Eu2O3 680ºC Eu2O3 830ºC
m
Figure 3. AFM images of mesoporous Eu2O3 films after heat treatment at 680 and 830 ◦C.
(This figure is in colour only in the electronic version)
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Nanotechnology 18 (2007) 055705 Y Castro et al
10
100
1000
10000
100000
10 20 30 40 50 60 70
2 theta
*
*
**
* ***
**
*
*
*
*
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*
9 0
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1 0 5
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1 2 0
1 2 5
1 3 0
4 6 4 7 4 8 4 9
2 T he ta
u.a.
(400)(440)
(a)(b) (440)
I (a
.u.)
Figure 4. XRD diagram of Eu2O3 films calcined at 680 and 830 ◦C.
Figure 5. Adsorption/desorption isotherms of water into the 680 ◦Ctreated film versus relative pressure of water in the atmosphere.
quality films. In figure 3, the pore structure is well resolvedwith an estimated wall thickness of 10 nm. This value canslightly differ from the real one due to a possible distortionof the piezoelectric device during scanning. In figure 3(b),the observed average grain size increases to 18 nm. Thisincrease is related to the diffusive sintering effect that leads tothe formation of larger crystalline particles. The geometry ofthe grains is non-spherical and the well designed pore structurehas been lost upon treatment to higher temperature. Theseobservations are in agreement with the SAXS and WAXSmeasurements.
In order to confirm the bcc crystal structure and todetermine the grain size, x-ray diffraction measurements wereperformed on mesoporous Eu2O3 films sintered at 680 and830 ◦C (figure 4). The results show numerous peaks associatedwith the silicon substrate (labelled with *) that mask themost intense peaks of the bcc Eu2O3 structure. However,two secondary peaks were detected that correspond to the(400) and (440) planes of the bcc phase. Because of thesmall amount of matter in the films, a low signal/noise ratiowas obtained. Moreover, a splitting of the peak at 47.2◦was observed, probably due to the presence of a slightlydistorted cubic structure, which seems to be reduced whenthe temperature increases. An accurate analysis of the grainsize was difficult to accomplish, but estimation was anywayperformed using the Scherrer’s formula from accumulatedXRD patterns (figure 4(b)): D = 0.9λ
β cos θB, where λ is the
wavelength, β is the quartz-standard-corrected full width athalf maximum (FWHM) of the Bragg peak and θB is the
Figure 6. Pore size distribution deduced from the EEP isotherm(adsorption branches) given in figure 5 for both temperatures oftreatment.
Table 1. Layer characteristics deduced from EEP measurement.Refractive indices (n) were measured for λ = 700 nm and atRH = 2%.
Bragg angle. The FWHM values were extracted from the XRDpattern simulation. Depending on the Gaussian/Lorentzianratio employed to fit the diffraction peaks, the calculatedparticle sizes ranged from 16 to 22 nm and 22 to 25 nm for thefilms at 680 and 830 ◦C, respectively. These values corroboratethose estimated by AFM and SAXS.
A detailed and accurate investigation of the porosity inboth samples was performed using EEP. Isotherms of wateradsorption/desorption within the film annealed at 680 ◦C aregiven in figure 5.
First, porosity in thin calcined films was confirmed by therefractive indices measured by ellipsometry under a relativehumidity of 2% (see table 1), which is much lower thanthat of the dense europium oxide material (i.e. n = 1.9 atλ = 500 nm). In figure 5, one clearly observes a suddenincrease in water uptake above P/P0 = 0.8, suggesting thatthe capillary condensation of water within pores takes placein a narrow range of humidity, which is characteristic of anarrow pore size distribution. The desorption branch (thereverse process corresponding to the departure of ‘liquid water’from the pores) takes place at slightly lower P/P0 valuesthan that of the adsorption branch. This narrow hysteresisbehaviour is typical of mesoporous structures with well definedpore dimensions and high accessibility. Similar hysteresismorphology was observed for the 830 ◦C treated films butwith the capillary condensation appearing at slightly higherrelative pressure. The corresponding pore size distributions,determined from adsorption branches, are plotted in figure 6for films treated at both temperatures, and reported in table 1.They were calculated using the Kelvin equation, which wasmodified to take into account the surface tension, the watercontact angle, and the presence of an adsorbed water layerat the pore surface. The geometric model applied for such a
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Nanotechnology 18 (2007) 055705 Y Castro et al
Figure 7. Scheme representing the variation of the structure duringthe heat treatment.
structure was cylindrical. Details of such calculations can befound in another paper [14]. When calcined at 680 ◦C, poredimensions are centred at 11 nm (±2 nm) in diameter, whileat 830 ◦C they enlarge to 13 nm with an equivalent dispersion.Interconnection between pores can be similarly deduced fromthe desorption branches and were centred at 9 and 10 nm for680 and 830 ◦C treatment respectively, which confirms the highaccessibility of the porous networks.
Such a pore enlargement is accompanied by a slightporosity increase, which is expected upon wall densification.Porosity of the Eu2O3 films treated at 680 and 830 ◦Cwas evaluated using the Bruggeman effective mediumapproximation (BEMA) model applied to ellipsometric data.Values of 45 and 49 vol% were founded for the 680 and 830 ◦Ctreatment respectively. Figure 7 illustrates the variation of thestructure during the heat treatment.
Finally, characteristic luminescence measurements, inparticular, fluorescence spectra and fluorescence decay curves,were carried out. Figure 8 shows the emission spectra ofthe mesostructured films annealed at 680 and 830 ◦C. Fiveemission bands located at around 580, 595, 615, 650 and700 nm, respectively attributed to the f–f transitions (5D0 →7F0,1,2,3,4) of the Eu3+ ions, can be observed.
From the fluorescence profiles, several features must bementioned. In general, electric dipole f–f transitions in free4f ions are parity forbidden, but become partially allowed bymixing with orbitals having different parities because of anodd crystal field component. In our samples, the apparitionof the forbidden 5D0 → 7F0 electric dipole transition band(�S �= 0 and J = 0/J ′ = 0) suggests that Eu3+ ionsare located in a coordination sphere with low symmetrysuch as Cnv, Cn or Cs [15]. Another interesting trend isthe high intensity of the band appearing at 615 nm. Itcorresponds to the ‘hypersensitive’ 5D0 → 7F2 electricdipole transition and its intensity depends on the surroundingsof the europium centres. Indeed, the ratio (labelled as Rin this paper) between the intensities of the 5D0 → 7F2
(‘hypersensitive’ transition) and 5D0 → 7F1 (parity-allowedtransition not affected by the symmetry of the Eu site) bandsis generally admitted as an ‘asymmetry parameter’ of theEu3+ coordination polyhedron. Taking into account that fora commercial Eu2O3 powder in which europium atoms are ina symmetric eightfold coordination R was estimated to be five,the value obtained for the annealed films (R ≈ 12) indicates
5D0 →7F2
5 D0→
7 F1
5 D0 →
7 F0 5D0→7F4
5D0→7F3Eu2O3 830ºC
Eu2O3 690ºC
Eu2O3
powder
550 600 650 700 750
Wavelength (nm)
Inte
nsity
(a.
u.)
Figure 8. . Emission spectra of a crystalline commercial Eu2O3
powder and the mesostructured thin films after heat treatment at 680and 830 ◦C, all of them normalized by the 5D0 → 7F1 band.
that there are Eu3+ ions in highly asymmetric coordinationspheres. This is in good agreement with the presence ofthe electric dipole 5D0 → 7F0 transition band. However, itis important to note that since the emission spectrum is anaverage of the fluorescence emitted by all the luminescentcentres, the existence of more symmetric sites is not discarded.In this sense, the unresolved inhomogeneous broadening ofthe emission bands would suggest the existence of differentlocal environments, but to go deeper into these questionsmeasurements at low temperature would be necessary. Thehigh asymmetry and the existence of several Eu sites are twotrends that have been commonly found in sol–gel-derived Eu-doped materials [16–20] but no information about mesoporousEu2O3 films has been reported until now. In these lattermaterials, the mesoporous network order is responsible forthe formation of anisotropic particles as a result of the walland pore uniform morphology within which nucleation andgrowth takes place. Therefore, it is reasonable to assume a highextent of the Eu2O3 grain boundaries and different distortedsites.
The corresponding luminescence decay curves wererecorded with fixed excitation at 465 nm (5D2 level) andemission at 615 nm (5D0 → 7F2 transition). They arerepresented in figure 9 together with the curve of a commercialEu2O3 powder taken as reference. The decay profile of theEu2O3 powder could be adjusted with a single-exponentialfunction, showing a lifetime value (τ) of 45 μs. Thisvalue is quite short compared to typical Eu-doped systemsused as efficient light emitters whose lifetimes are in themillisecond scale, but is characteristic of materials with a highEu concentration in which non-radiative mechanisms governthe relaxation processes. These phenomena are the well known‘concentration quenching’ and are attributed to energy transferbetween Eu3+ ions located at short distances which finallyinteract with quenching species (defects, surface. . .). The
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Nanotechnology 18 (2007) 055705 Y Castro et al
Figure 9. Room temperature decay profiles of a commercial Eu2O3
powder and mesostructured thin films after heat treatment at 680 and830 ◦C, recorded with fixed excitation at 465 nm (5D2) and emissionat 615 nm (5D0 → 7F2 transition).
decay curve of the mesoporous film treated at 680 ◦C shows avery fast relaxation with a clearly non-exponential behaviour.In this case, several lifetime values (τn) can be calculatedaccording to the equation [21]: τn = I0/2.73n. The followingvalues were obtained for this film: τ1 = 1.6 μs, τ2 = 3.6 μsand τ3 = 6.6 μs. These short lifetime values as well asthe non-exponential character could be mainly ascribed tothe disorder affecting the sites in which the europium atomsare accommodated around the mesopores. The thin Eu2O3
walls and the high porosity (45 vol%) of the film heat treatedat 680 ◦C are probably responsible for the distortion in thecrystalline structure, which would explain the existence ofmultiple Eu sites and very short Eu–Eu distances, all of themcontributing to the non-radiative relaxation. In the mesoporousfilm annealed at 830 ◦C, a double contribution has beendetected. Its decay profile can be fitted to a non-exponentialbehaviour for the shorter times, with τ1 = 2.8 μs, τ2 = 5.8 μsand τ3 = 10.3 μs and a single-exponential function for thelonger times, with a τ = 47 μs (comparable to the value ofthe Eu2O3 powder). This latter single-exponential relaxationhas not been detected in the sample calcined at 680 ◦C sinceluminescence intensity vanishes before a possible long lifetimecontribution. Its presence at 830 ◦C can be attributed tothe sintering of crystalline Eu2O3 nanodomains in whichEu3+ ions are located in crystallographic atomic positions.However, the non-exponential contribution indicates that animportant number of the Eu3+ ions are still located in highlydistorted sites around the grain boundaries. In conclusion, itcan be said that the Eu3+ ions in the mesostructured Eu2O3
thin films present an optical response characteristic for thiskind of luminescent system, and are in good agreement withthe structural organization of the material. For enhancedfluorescent materials, mesoporous Eu-doped systems are underinvestigation in our group (Eu–Y2O3 exhibits lifetime values of2 ms [22]) in order to improve the properties of the other Eu-activated photonic materials such as Eu–TiO2 which showedlifetimes of around 500 μs [23].
4. Conclusions
In the present study europium was used as a good model tomaster the synthesis and the processing of new periodicallyorganized and nanocrystalline mesoporous films based ontrivalent lanthanide oxides.
For the first time, nanocrystalline rare earth oxide(Eu2O3) mesoporous thin films were successfully preparedby the EISA method. They exhibited good crystallizationat nanometric scale as shown by HRTEM analysis. Meso-organization of well calibrated pores of around 10–13 nm indiameter and a correlation distance of ∼25 nm between themwere corroborated by AFM measurements and ellipsometry.Photoluminescence properties have been studied to probe thematerial structure. Optical features and the non-exponentialresponse of the decay profiles were attributed to Eu3+ ionslocated in highly asymmetric environments. In view of thecrystallinity of the Eu2O3 domains, these asymmetric siteswould correspond to Eu3+ ions in distorted coordinationspheres of the grain boundaries, and would decrease in numberduring thickening of the mesoporous network wall as a result ofparticle sintering. We believe that this strategy of synthesis canbe extended to other mesostructured nanocrystalline rare earthoxides with interesting properties for advanced applications inthe field of catalysis, magnetism, etc.
Acknowledgments
The authors would like to thank Pr Markus Antonietti fromthe MPI of Golm for providing the KLE block copolymers,the European Network of Excellence FAME and the CNRS.The Centre of Electronic Microscopy of Orleans (France) isacknowledged for the TEM analysis. YC and BJ wish to thankthe Spanish Ministry of Education and Culture for its financialsupport (EX-2004-0324 and EX-2004-0310).
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